1. Field of Invention
This invention relates to a communication system and a communication control method, wherein stations are connected to buses and communicate with each other through the buses.
2. Description of the Prior Art
A computing system is typically divided into CPU (central processing unit) sections, storage unit sections, input/output interface sections, etc, and a plurality of printed wire boards comprising the sections are installed in such system. The printed wire boards are interconnected by connecting connectors attached to the boards through a bus. A board provided with connectors and a bus is generally called a backplane. A communication system is built using a backplane to enable printed wire boards in the system to communicate with each other. The prior art and its problems found in a communication system where backplanes are adopted, are as follows:
In a communication system comprising a master station and a slave station with a bus interconnecting the stations is often duplicated in order to increase communication reliability. In such a dual redundant bus system, two redundant buses are used alternately as long as both buses are in normal condition. If one of the buses should fail, the other normal bus is used to continue communication. Meanwhile, concurrent communication is carried out in order to check periodically whether or not the failed bus has recovered.
Another problem is that the master-slave communication does not take place unless a processor in the master station is aware of the status of the redundant buses. More specifically, the processor must be aware of which of the two redundant buses is the active bus or the standby bus.
A further problem is that control must be carried out to switch from one bus to the other bus when either of the buses fails. A further problem is that the integrity of the transferred data is checked by adding check bits to the data. Although this checking procedure using check bits can examine the integrity of the data on the bus, it cannot examine the integrity of the data in areas other than the bus, such as bus interfaces, bridges, and repeaters.
Also, in communication systems, there are various reasons why the waveform of a bus signal may become distorted, as described below. FIG. 1 shows a conventional communication system wherein a plurality of units 21 to 2n are connected to a bus 1 in a multidrop configuration. Units 21 to 2n communicate with each other through bus 1. FIG. 2 shows an equivalent circuit of bus 1, wherein bus 1 has its own inductance L and stray capacitance C. When any of the units 21-2n is connected to bus 1, the circuit impedance decreases because of the capacitance component C of the unit itself. Accordingly, a signal transferred through bus 1 to the unit 21 . . . 2n is reflected back to points where other units 21 . . . 2n are connected. For example, if a signal is sent from unit 21 to unit 22 in FIG. 1, reflected signals occur at the connection points of units 22 to 2n to bus 1.
FIG. 3 shows the waveform of a signal at point B of FIG. 1. Signal reflected by units 23 to 2n reach point B before the signal received by unit 22 changes from a high level state to a low level state. As a result, the reflected signals from units 23 to 2n are superposed with the received signal, as shown in FIG. 3, thus increasing the degree of waveform distortion. This may cause receiving unit 22 to malfunction. In the example of FIG. 3, the magnitude of the superposed reflected signals exceeds the low level threshold.
In order to avoid this problem, the following restrictions are applied. in the prior art: (A) Special devices having low capacitances are used with the units. (B) The number of connected units is reduced. It is desired to control the effects of such reflected signals without being limited by these prior art restrictions.
The waveform of a bus signal may also become distorted in the following manner. In a communication system, the transmitter circuit of a unit is provided with a driver IC (integrated circuit) that sends out signals to a bus. If any one bit, among a plurality of bits inputted to the driver IC, is kept static and all of the other bits are switched at the same time, the ground potential of the driver IC increases. This phenomenon is known as “ground bounce”, and noise may be induced at the static bit due to the effects of “ground bounce”. This noise is also known as “simultaneous switching noise”, and faulty data may be transferred due to the “simultaneous switching noise”. Once the “ground bounce” occurs, it takes some time for the ground potential to return to zero. This results in a disadvantageous increase in the communication delay time. It is desirable to reduce the effects of “ground bounce” which plagues prior art systems and methods.
FIG. 4 shows a standard communication system which has another problem. In FIG. 4, a transmitter circuit 11 and a receiver circuit 12 are connected to a transmission line 10 which constitutes a bus. Data is transferred from transmitter circuit 11 to receiver circuit 12 through transmission line 10. Transmitter circuit 11 and receiver circuit 12 operate on asynchronous clocks having different phases. Before any signal transfer can be carried out in the communication system, data transmitted using the clock in the transmitter circuit must be somehow synchronized with the clock in the receiver circuit. There are certain difficulties existing as a result.
If data needs to be transferred using start stop synchronization that transmits data only, this synchronization is achieved only by using a clock which is faster than the data transfer rate for the receiver to sample the data. Normally, a high speed clock having a frequency which is approximately 16 times the data transfer rate is used.
On the other hand, if data needs to be transferred using clock synchronization that sends data together with a clock signal, this synchronization is achieved by writing the data once into a FIFO circuit in the receiver circuit using the transmitted clock signal, and then reading the data from the FIFO circuit using the clock in the receiver circuit.
Disadvantageously, data transfer based on start stop synchronization requires that the receiver circuit be provided with a clock that operates at speeds higher than the data transmission rate. As a result data transmission rate must be lower than the frequency of the clock available for the receiver circuit. For this reason, in the prior art, high speed signal transfer has been difficult to achieve.
Also, disadvantageously, data transfer based on clock synchronization requires that the data be written once into a FIFO circuit in the receiver circuit using the transmitted clock signal. For this reason, faulty data may be written into the FIFO circuit if the waveform of a received clock signal is distorted. The waveform of signals that propagate between circuits connected to the transmission line or bus are distorted due to the capacitive load of the transmission line or due to the effects of the noise that enters the transmission line. Thus, in the prior art, it is difficult to achieve high speed, consistent signal transfer.
Moreover, in the art, where two or more bus masters share the same system resources, such as storage units, through a common bus, concurrent or simultaneous requests from the respective bus masters to use the same bus cause conflicting demands. If this happens, some method of control must be used to decide which bus master should get first use of the bus. Bus arbitration is carried out for this purpose.
FIG. 5 shows a conventional communication system, wherein bus masters 31 to 3n are connected to a data bus 42 and an arbitration bus 43. A slave 44 is, for example, a storage unit and is connected to the data bus 42. An arbiter 45 is incorporated in each of the bus masters 31 to 3n. The arbiter 45, after having executed arbitration procedure using arbitration bus 43, permits the bus master that has acquired the right to use data bus 42 to do so. The bus master that has acquired the right to use the data bus 42 gains access to slave 44.
FIG. 6 shows operation of the embodiment of FIG. 5, wherein bus masters 31 and 32 acquire the right to use data bus 42 in succession in the order of the bus master 31, then bus master 32, and the bus master 31. In this process, an arbitration procedure using arbitration bus 43 takes place each time the right to use is acquired. However, when a bus master then has acquired the right to first use of bus 42 wants to again use the data bus 42 in succession, the arbitration action takes place even if no other bus master requests use of the data bus 42. In the example of FIG. 6, where bus master 32 uses the data bus 42 in two consecutive rounds, the arbitration action takes place each time the bus master acquires the right of use.
This prior method of arbitration involves a waste of time and leads to the problem of performance degradation. The amount of wasted time increases especially when one particular bus master alone uses the data bus more often than the other bus masters.
Thus, as described above, the prior art has many problems and defects which need improvement.